A High Voltage Direct Current (HVDC) power distribution network or an HVDC power transmission or distribution system uses direct current (DC) for the transmission or distribution of electrical power, in contrast to the more common alternating current (AC) systems. For long-distance transmission or distribution, HVDC systems may be less expensive and may suffer lower electrical losses. However, HVDC systems may also be used for short-distance transmission or distribution. In general, an HVDC power transmission or distribution system comprises at least one direct current transmission or distribution line, e.g. a long-distance HVDC link or cable for carrying direct current a long distance, e.g. under sea, and converters, or converter stations, for converting alternating current to direct current for input to the HVDC power transmission or distribution system and converters for converting direct current back to alternating current for input to a high voltage AC power system. An HVDC power transmission or distribution system may also be used as a back-to-back system for interconnecting two asyn-chronous AC networks, which may be remote to or in the proximity of one another. In a back-to-back system, the HVDC power transmission or distribution system may take electrical power from a first AC network, convert it into to direct current and transmit it to a second AC network, which may be remote to or in the proximity of the first AC network, where direct current is converted back to alternating current for input to the second AC network.
A converter connected to an AC power system, for example a converter included in a converter plant for high-voltage direct current, generates, by its principle of operation, harmonic currents on its AC side and harmonic voltages on its DC side. In this context, in principle, only harmonics to the fundamental frequency of the AC system of the orders n=k×p±1 occur on the AC side and of the orders n=k×p on the DC side, p being the pulse number of the converter and k being a positive integer. Harmonics of other orders may also occur in power systems of this kind, caused by, for example, un-symmetries between the phases of the AC system.
To reduce the stresses on components, e.g. generators and transformers, included in the electrical power transmission or distribution system, e.g. in the form of heating of the components, and originating from the harmonics, and to fulfill the requirements made on the effect on the electrical power transmission or distribution system and telecommunication disturbances, filters are generally installed to limit the propagation of the disturbances in the electrical power transmission or distribution system. Harmonics of a lower order, e.g. harmonics corresponding to k=1 and for 6-pulse converters also harmonics corresponding to k=2, are generally filtered through filters tuned to these harmonics, whereas harmonics of a higher order may be filtered through a high-pass filter. The filters are composed of pas-sive components, and during the dimensioning, it may also be taken into consider-ation that the filters on the AC side are to serve as members for generating reactive power. In general, however, the requirements for generation of reactive power in a converter plant for high voltage direct power may require the installation of one or more further high voltage capacitor banks on the AC side. In certain cases, it may be necessary to install tuned filters and high-pass filters also on the DC side of the converter. In a converter plant for high voltage direct current, the filters and the capacitor banks constitute plant components which essentially influence the function, volume and cost of the plant.
In general, the tuned filters are designed as series-resonance circuits, comprising capacitive, inductive and sometimes also resistive impedance elements, tuned such that, at one or more of the harmonic frequencies expected in the electrical power transmission or distribution system, they are to exhibit purely resistive impedance.
In narrow-band filters, a small change of the reactance of an impedance element included in the filter may cause a considerable deterioration of the function of the filter. Such a change may, for example, be caused by a fault in one part of a capacitive impedance element.
Variations in the system frequency and drift in component values, caused by, for example, temperature variations or aging, makes it in general difficult to maintain a satisfactory, or exact, tuning although no direct faults occur in the filter, which in turn results in an impaired electric power transmission or distribution and an impaired control thereof. One way to address variations in the system frequency and variations in component values is to make the filters broad enough to cope with the variations. It has also been proposed to provide tunable filters which provide an adjustment of the resonance frequency or frequencies of the filter.
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